There exist methods for two-dimensional latitude/longitude position location systems using radio signals. In use are terrestrial systems such as Loran C and Omega and satellite-based systems such as the Transit system and Global Positioning System (GPS).
Initially devised in 1974, GPS is widely used for position location, navigation, surveying, and time transfer. The GPS system is based on a constellation of 24 on-orbit satellites in sub-synchronous 12 hour circular, inclined orbits. Each satellite carries a precision atomic clocks and transmits a pseudo-noise signal, which can be precisely tracked to determine pseudo-range. By tracking 4 or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996.
GPS has revolutionized the technology of navigation and position location. However in some situations, the effectiveness of GPS is limited because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of −160 dBW as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of line-of-sight blockage or while the receiver is inside a building.
In recent years, there has been a rollout of digital television in Asia, Europe and the Americas. Some of the primary standards around the world are ATSC (e.g. United States), DVB (e.g. Europe) and ISDB (e.g. Japan). As of February 2001, approximately 1200 DTV construction permits for US DTV stations had been acted on by the FCC. Over 1600 DTV transmitters are expected in the United States. Other regions are implementing similar DTV systems. The Japan Broadcasting Corp. (NHK) has defined a terrestrial DTV signal for Japan, referred to herein as the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) signal. These new DTV signals permit multiple TV signals to be transmitted in the assigned 6 MHz radio channel. All of these different television standards employ an embedded synchronization code which is used to probe the transmission channel and mitigate the effects of multipath in a digital TV receiver. In order to be effective for channel modeling and multipath mitigation, these synchronization codes have wide bandwidths, narrow time autocorrelation functions, and high power levels. The above-stated features make the synchronization codes ideal for positioning, in particular indoors where multipath effects are severe and GPS signals may not penetrate. In addition, analog television broadcasts have also started in recent years to insert into their broadcasts a synchronization code termed the Ghost-Canceling Reference (GCR), which is used for multipath mitigation on analog signals in TV receivers that digitize the signal. Consequently, the GCR can also be used for precise ranging. Other test signals inserted in the analog broadcasts, such as the multiburst signal, may also be used for position determination.
There is a proposed system for using conventional analog National Television System Committee (NTSC) television signals to determine position. This proposal is found in a U.S. Patent entitled “Location Determination System And Method Using Television Broadcast Signals,” U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the technique described the use of the horizontal and vertical synchronization pulses which were intended only for relatively crude synchronization of the TV set sweep circuitry, and thus will limit the level of positioning accuracy or reliability of the disclosed location technology. Further, the Federal Communication Commission (FCC) is considering decommissioning NTSC transmitters and reassigning their frequency spectrum allocations.
Additionally, there are demands to incorporate the types of services that DTV can provide within the mobile user devices. For example, efforts are underway in Korea and Japan to generate cellular handsets which include television tuners for the purpose of receiving television on mobile devices as well as conveying data using the DTV channel. Further, besides enabling High Definition Television, the new Japanese ISDB-T DTV signals are also designed for use by the mobile user, and have the ability to transmit data reliably at reduced rates to small handheld devices. Cellular phone and PDA devices will soon be released in Asia which included TV micro-tuners for the purpose of receiving data broadcast over the DTV spectrum. In a device which already includes a TV micro-tuner, there's a need for adding a TV/GPS position location capability.